A scientific team used one of Canada's RADARSAT satellites to map New Orleans and found that most of the city sank about a quarter inch (0.06 centimeter) annually in the three years leading up to Katrina's landfall.

But many areas, including some of the levees designed to hold back the waters of the Gulf of Mexico, sank at four or five times that rate.

"What we found is that some of the levee failures in New Orleans were [in] places where subsidence was highest," University of Miami geophysicist Tim Dixon told the Reuters news service.

The data suggest that some levees could be 3 feet (0.9 meter) lower than when they were built some 40 years ago.

The Mississippi River Gulf Outletthe levee-protected canal notorious for its role in the flooding that followed Hurricane Katrinahas sunk more than three feet, the team's study reveals.

"That levee certainly was much lower than what it was originally designed for," said Louisiana State University geologist Roy Dokka, who took part in the research.

Rapidly sinking ground under a levee could cause structural weaknesses in the levee system, he explains.

But sinking levees aren't the only cause for concern raised by the research.